Past industrial activities have left behind degraded areas, where different contaminants are present in the soil, which is for this reason often classified as hazardous waste. Such soils contain hazardous substances and various species of trace elements in both mobile and bioavailable form. Degraded areas represent "old burdens" which need to be rehabilitated in the most practical, economical, and environmentally acceptable way. One such old burden in Slovenia involves the area of the Old Zinc-Works near the town centre of Celje. There an area of 17 ha is covered by more than 1.5 M m3 of contaminated soil, which has been classified as hazardous waste according to the Slovenian legislation since it contains large quantities of water soluble species of different trace elements (As, Zn, Pb, Cd, Cu, Mo), as well as SO42-. An innovative remediation and rehabilitation method has been developed for the treatment of this heavily contaminated soil material in situ, and has been validated in the real case treatment of the most contaminated soil from the above-mentioned old zinc-works area. An immobilization approach is used, where the bioavailable and mobile species of the trace elements are chemically and physically incorporated into a newly formed composite, which is a mixture of crushed and homogenised contaminated soil to which ash from paper sludge incineration, as well as electric arc furnace slag from secondary metallurgical processes, is added. After establishing the optimum moisture content, the composite is placed as compacted layers, i.e. as part of engineering fill. Due to the high degree of compaction and pozzolanic or hydraulic activity of the additives, a cemented matrix is formed. In this process the contaminants are either physically microencapsulated, or chemically adsorbed onto the surface and/or incorporated into the crystalline lattice of the calcium silicate hydrate phases. The hydration products are similar to those of Portland cement. The alkaline pH values of the pore solution favour the formation of low soluble hydrolysis products of most of the trace elements. Hydraulic conductivity, too, is very low, which means that very little water is percolated, and that the rate of transport of chemical species of trace elements is thus very slow.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 2159207The process according to the invention proposes a new and inventive approach for resolving of problems related to environmental pollution. Such technology enables restoration of seriously degraded areas resulting from long-term industrial activities. These are mainly areas of iron and steel processing plants or metallurgic-chemical plants, where in particular inorganic pollutants were released to the air, water and soil, and where huge amounts of industrial waste were deposited. Although industrial activities were terminated, these areas remain not only degraded and inoperative but also represent a permanent problem of pollution of the environment due to surface and underground water streams and also due to emission of powder particles. The invention belongs to working operations, namely to methods of remediation of contaminated soil, by which the latter is transformed into less harmful substance. The purpose of the invention is to enable economically efficient obtaining of construction material, which should be chemically neutral, inert and acceptable for the environment and human health, by utilization of the contaminated soil. In this construction material the concentration of water soluble compounds of heavy metals do not exceed pre-determined values, namely: As ( 0,50 mg/kg of dry matter, Cd ( 0,04 mg/kg of dry matter, Pb ( 0,50 mg/kg of dry matter and Zn ( 4,00 mg/kg of dry matter in such obtained construction material. Said material is obtained from a contaminated soil containing water soluble chemical compounds of heavy metals, which exceeds the previously mentioned values, namely: As = 1,00 – 10,00 mg/kg of dry matter, Cd = 0,10 – 1,00 mg/kg of dry matter, Pb = 1,00 – 10,00 mg/kg of dry matter and Zn = 5,00 – 50,00 mg/kg of dry matter in said contaminated soil. The process according the invention comprises the following steps: 1) Preparation of the contaminated soil includes excavation, separation of dry particles of contaminated soil by means of sieving and crushing to granulation below 32 mm and temporarily deposition on horizontally layered depot. 2) This is followed by homogenization of contaminated soil and mixing with a dispersion of Fe-nanoparticles. 3) Next step includes addition of bentonite clay in powder form to said mixture of soil and Fe-nanoparticles and homogenization of such obtained mixture. 4) Calcareous fly ash is added to the previously obtained mixture, which is followed by mixing and optionally adjustment of moisture to optimum value. 5) Finally in situ or ex situ application of such obtained material is performed by spreading it in to layers having thickness up to 30 cm and compacting such established layer by means of suitable building machinery. The average density of such compacted layer is ≥ 95 % of the density of the mixture, which is obtained by means of modified Proctor method (SIST EN 13286-2:2010/AC:2013). Said dispersion of Fe-nanoparticles is added to the soil in step 2) in amount up to 8 % of maghemite particles, or up to 3 % of nanoscale Zero-Valent Iron per weight of a dry contaminated soil. Said bentonite clay is added in said step 3) in amount up to 10 % per weight of a dry contaminated soil. Whenever the content of clay fraction or particles with granulation ≤ 0,002 in the soil exceeds 40 % per weight of the dry contaminated soil, then the zeolitic tuff in powder form is added to the mixture of contaminated soil and dispersion of Fe-nanoparticles instead of said bentonite clay. The added quantity of said calcareous fly ash amounts up to 20 % per weight of dry contaminated soil. Remediation leads to immobilization of metals that is proved with the standardized test for determination of content of water soluble species of metals (SIST EN 1744-3:2002). Concentration of the pollutant in the leachate determines a degree of immobilization of harmful metals in remediated material, which shall not exceed the previously mentioned values.
F.09 Development of a new technological process or technology
COBISS.SI-ID: 2163303Contamination of the soil with trace elements in the concentrations and chemical forms that pose a threat to human health and the environment, is a major global problem. It is estimated that in the European Union is located 250,000 contaminated sites, of whom 34.8% are contaminated with trace elements. The Celje basin is considered one of the oldest industrial areas in Slovenia, where company Cinkarna Celje has a particularly long operational history. The latter was originally located in the vicinity of town centre, in the area, which is known today as the Old Zinc-works site. Analyses of the soil from this site have shown that it has properties of hazardous wastes according to the valid Slovenian legislation. It exceeded the limit values for some trace elements (Zn, Pb, Cd), as well as SO42- and organic substances. The State and the Municipality of Celje were thus forced to seek the best possible solution, both from an environmental as well as economical point of view. Approach with immobilization of contaminants in situ was chosen as the most suitable option for the remediation of contaminated soil in the Old Zinc-works site. Immobilization approach methods include mixing of contaminated soil with various additives, which enable physical (encapsulation) and chemical processes (ion exchange, sorption, oxidation/reduction reactions, the formation of solid solutions, precipitation) responsible for immobilization of hazardous chemical species of trace elements. As additives can be used different materials, such as Portland cement, lime, phosphates, clay, iron compounds, recycled materials (fly ash, steel slag, red mud, humus substances, etc.). Selection of a portland cement as an additive is common, but its use was in this case not justified, because of the presence of contaminants that would inhibit the formation of the cemented matrix. The most promising option was the use of reactive fly ashes of local industries and thermal power plants. Therefore it was developed the remediation method in which soil mixed with fly ash was processed into a construction composite, which was placed in the Old Zinc-work site as the structural fill. Composite consisted of a paper mill ash from company Vipap Videm (from the combustion of paper sludge and a little wood biomass) and contaminated soil from the Old Zinc-works site. It was a new building material in which all pollutants were effectively immobilized. This was confirmed by the results of leaching test and electron microscopy. For remediation process efficiency, it was important to conduct it properly. Contaminated soil needed to be homogenized and mixed with an appropriate amount of ash at the optimal moisture content, which was determined by modified Proctor test. This was followed by the compaction of composite in layers, according to the geotechnical principles. Only in this way it was achieved the highest possible bulk density and thus low water permeability, which was a guarantee for the effective physical and chemical immobilization of contaminants.
F.09 Development of a new technological process or technology
COBISS.SI-ID: 2165095